1. Field of the Invention
The present invention relates to an optical pickup apparatus.
2. Description of the Related Art
A CD (Compact Disc) is known as a medium capable of recording/reproducing information by means of infrared laser light. A DVD (Digital Versatile Disc) is known as a medium using red laser light. Also proposed are optical discs such as an HD DVD (High Definition DVD) or Blu-ray Disc (registered trademark) using bluish-violet laser light, capable of higher density recording than the above optical discs. In order to record/reproduce information on/from two different types of optical discs using laser lights of different wavelengths, some pickup apparatuses include a single lens holder mounted with an objective lens focusing red or infrared laser light for CD, DVD, etc., and with an objective lens focusing bluish-violet laser light for HD DVD, Blu-ray Disc, etc. (see., e.g., International Publication WO98/02874).
A disposition of the two objective lenses of the optical pickup apparatus relative to the optical disc can be e.g., a disposition along the direction of the radius (hereinafter, referred to as radial direction) of the optical disc or a disposition along the direction of the tangent (hereinafter, referred to as tangential direction) of tracks concentrically formed on the optical disc. A tracking control is commonly performed to cause laser light to follow a target track in the information recording/reproduction on/from the optical disc. With respect to the tracking control, there are employed in general a differential push-pull method, a three-beam method, etc., using three different laser lights (0th-order light, ±1st-order diffracted lights) that are obtained by diffracting laser lights by means of e.g., a diffraction grating. In the differential push-pull method for example, +1st-order diffracted light and −1st-order diffracted light are applied line-symmetrically with respect to the tangential direction of an information recording/reproduction target track, thereby enabling better tracking control than in the conventional push-pull method, etc. In the case of arranging the two objective lenses in the radial direction, both the objective lenses can apply +1st-order diffracted light and −1st-order diffracted light line-symmetrically with respect to the tangential direction of the information recording/reproduction target track. On the other hand, in the case of arranging the two objective lenses in the tangential direction, when one objective lens can apply +1st-order diffracted light and −1st-order diffracted light line-symmetrically with respect to the tangential direction of the information recording/reproduction target track, the other objective lens applies +1st-order diffracted light and the −1st-order diffracted light line-asymmetrically with respect to the information recording/reproduction subject track. This may result in fluctuations in amplitude of tracking error signals corresponding to the reflected lights of 0th-order light and ±1st-order diffracted lights, making infeasible the tracking control based on the differential push-pull method. Therefore, in the case of arranging the two objective lenses in the tangential direction, another push-pull method, etc., based on 0th-order reflected light is employed which may be inferior in tracking control to the differential push-pull method.
Thus, when mounting the two objective lenses on the single lens holder of the optical pickup apparatus, it is desirable to choose the disposition in the radial direction ensuring better tracking control based on the differential push-pull method for example.
In the case of disposing the two objective lenses in the radial direction, when for example the radially outside objective lens focuses laser light on a track of an information recording layer of the optical disc, the radially inside objective lens lies further radially inside than the case of the two objective lenses disposed in the tangential direction or than the case of the lens holder mounted with only a single objective lens.
As exemplarily shown in FIG. 7, when a radially outside objective lens 902 focuses laser light on, e.g., a track of a further radially inside area (hereinafter, referred to as innermost circumference of the information recording layer) than a read-in area (or PCA (Power Calibration Area)) of the information recording layer 22 of the optical disc 2, a radially inside objective lens 901 lies most inside in the radial direction. FIG. 7 is a side view of the radial disposition of the two objective lenses 901 and 902. In the exemplary view of FIG. 7, the objective lenses 901 and 902 are held by a lens holder 900 which in turn is housed in a radially shiftable housing 900 via suspension wires 920a. In the exemplary view of FIG. 7, a turntable 40 is fitted to a rotor 50 having a diameter smaller than that of the turntable 40.
As exemplarily shown in FIG. 7, even if the lens holder 900, the housing 930, etc., have specifications not requiring them to move to the further radially inside area than the read-in area (or PCA), the objective lens 901 at its lens face may butt against the turntable 40 when moving to the innermost circumference of the information recording layer 22, though a wire retaining member 900a, etc., can avoid butting thereagainst. For this reason, the objective lens 902 cannot access the innermost circumference of the information recording layer 22, making it infeasible to perform recording/reproduction on/from tracks in this area.
If reducing the diameters of the objective lenses 901 and 902 to avoid the above butting, then the effective diameter for gathering laser light also becomes smaller accordingly. As a result, when the housing 930 shifts in the radial direction to follow the disc eccentricity, etc., there may arise, e.g., a reduction in the light quantity of laser light focused on an optical disc 2 or a degradation in amplitude of a signal (e.g., reproduction signal) corresponding to the reflected light of laser light. If reducing e.g., the distance between the objective lenses 901 and 902, and the length between right ends of the lens holder 900 and of the objective lens 901 in FIG. 7 to avoid the butting, then the rigidity of the lens holder 900 becomes lower, which may induce a resonant mode unnecessary for an actuator (not shown).
If setting the level of the objective lens 901 lower than the level of the objective lens 902 with a step in the focus direction relative to the top face of the lens holder 900 in FIG. 7 to avoid the butting, the lens face of the radially outside objective lens 902 may come nearer to the disc surface of the optical disc 2 by a length corresponding to the step when the radially inside objective lens 901 is in use. This means that the working distance (WD) of the radially outside objective lens 902 becomes substantially short when the radially inside objective lens 901 is being used. For this reason, if the focus servo is out of action, then the lens face of the objective lens 902 may possibly butt against and damage the disc surface of the optical disc 2.